I remember seeing a thread on this a few months back, and was able to get facetime working on my unlimited plan for about a day. As I recall the rollout of FT on unlimited was an error on AT&T's part and they ended up correcting it.

For some reason, some people can use it over their unlimited plans and others can't. I called AT&T a little while ago. The rep told me that it can't be used if you have an unlimited data plan. But, some people here are able to use it over theirs. Strange...

I first noticed this feature started working around Thanksgiving and then the option went away right after. It seems to have returned right before Christmas, so my guess is this is just some gesture by AT&T for the holidays and I don't expect it to last much longer

The ones without it seem to be those of us with the grandfathered Unlimited data plans.

I've always had a tiered data plan with my iPhone 5, and Facetime without wifi just started working recently. I tried it a millions times before and when I tried to switch it on I got a pop up message saying to activate it I would have to call AT&T or visit the website.

AT&T appears to be quietly extending FaceTime-Over-Cellular capabilities to all their iPhone customers. Earlier this month, AT&T announced that they would allow any customer "with an LTE device" on a "tiered data plan" to use iOS 6's FaceTime over Cellular feature.

This, however, excluded a number of customers, including those with grandfathered unlimited data plans as well as those with the iPhones 4S. (Note, iOS 6, itself, can only supports FaceTime-over-Cellular on the 4S, 5 and iPad 3rd Gen or higher)

Tonight, Forum user kre62 noticed that both his iPhone 4S and iPhone 5 with unlimited AT&T data plans now support FaceTime over cellular.This finding has been confirmed by a numberofusers in this discussion thread -- though some users have been unable to activate it so it may still be rolling out to some areas. AT&T had said the original rollout for tiered plans could take 8-10 weeks. Most of the positive responses have been those with the iPhone 5 and grandfathered unlimited data, but the original poster also had luck with an iPhone 4S.

Some users have had to completely restart their phones for the service to go into effect. There still remains a possibility that this is an oversight on AT&T's part. We are reaching out to AT&T for clarification on their policy.

I have the 2GB plan, it works sometimes and others doesn't. When I flip the switch it says I have to call ATT to have the proper rate plan. Power cycling the phone turns the setting back on. So anytime I can't facetime over cellular, I have to powercycle and then it works. Very odd, I don't know if its an issue when the phone associates with a different tower or what but its hit or miss. Very frustrating sometimes.